Month

A month (from the Latin mensis) is each of the twelve periods of time, between 28 and 31 days, into which the year is divided.

The duration was established in such a way that the months of 30 and 31 days were intercalated, with the exception of February, which kept its original duration of 28 days for religious reasons (see leap year).

Months in the Julian and Gregorian calendars

The Gregorian calendar, like the Julian calendar before it, has twelve months:

A mnemonic code to remember the length of the months is to hold both fists together with the index knuckle of the left hand next to the index knuckle of the right hand. Each knuckle represents a 31-day month, one space between knuckles represents a short month, and January is the small knuckle on the left hand.

Origin

The months of the Gregorian and Julian calendars come from the Roman Calendar. The names of the first six are related to their mythology and the last ones to their original order number (which started in March). The primitive Roman calendar, according to the tradition collected by Tito Livio Livio and Plutarch, was 10 months, to which King Numa Pompilius (716 BC - 674 BC?) added two more. In the time of Julius Caesar, other modifications were made: the month in which the year began was changed, henceforth it will be January, but not for all cases, only for the beginning of the Roman consular period, and one month will take the name of July in homage to the promoter of change; his heir and successor, César Augusto will name the following month; August.

The famous mnemonic Thirty days is September is a common way of teaching the length of months in the English-speaking world. The knuckles of the four fingers of the hand and the spaces between them can be used to remember the length of the months. When making a fist, each month will be numbered as it progresses in the hand. All the months that arrive in a knuckle have a duration of 31 days and those that arrive between them have a duration of 30 days, with the variable February being the remembered exception. When you get to the knuckle of the index finger (July), move to the first knuckle of the other fist, hold it next to the first (or return to the first knuckle), and continue with August. This physical mnemonic has been taught to elementary school students for many decades, if not centuries.

This cyclical pattern of the length of the months coincides with the musical keyboard's alternation of wide white keys (31 days) and narrow black keys (30 days). The note fa corresponds to January, the note F sharp corresponds to February, the exceptional month of 28–29 days, and so on.

The month in astronomy

The month is an astronomical unit of time, used in the calendar as the period it takes the Moon to go around the Earth. However, the movement of the Moon in its orbit is very complicated and its period is not constant. Furthermore, in many cultures (as in the Hebrew and Muslim calendars) the beginning of the month coincides with the first appearance of the crescent new Moon after sunset above the western horizon. The date and time of this actual observation depend on the exact geographic longitude, as well as latitude, atmospheric conditions, observers' visual care, etc. Consequently, it is not possible to accurately predict the beginning and length of months in these calendars. Most Jews follow a pre-calculated calendar.

Synodic month

The traditional concept arises with the phase of the moon cycle. The synodic month or lunation is the period that elapses between two same consecutive phases of the Moon, its approximate duration being 29.53 days. The cause of the phases of the Moon is that we see the part of the Moon that is illuminated by the Sun and this depends on its relative position with respect to the Sun (seen from Earth). Since the Earth revolves around the Sun, the Moon takes extra time (after completing one sidereal month) to return to the same position with respect to the Sun. This longer period is called synodic. Due to disturbances in the orbits of the Earth and Moon, the actual time between lunations can vary between approximately 29.27 and 29.83 days.

Archaeological finds show that time was already counted in the Paleolithic using the phases of the Moon. Likewise, the synodic month is still the basis of many calendars.

Sidereal month

The actual period of the Moon's orbit taking the fixed stars as reference is called the sidereal month, because it is the time it takes for the Moon to return to the same position towards the fixed stars in the celestial sphere. It is worth approximately 27 1/3 days on average. This type of month has appeared among cultures in the Middle East, India, and China as follows: they divided the sky into 28 parts, characterizing a constellation, for each day of the month in a way that follows the footprint that the Moon leaves among the stars.

While Orthodox Islam and Karaite Jews still rely on actual observations of the Moon, reliance on astronomical calculations and tabular methods is becoming more common in practice.

Tropical month

It is customary to specify positions of celestial bodies with respect to the vernal equinox. Due to the precession of the equinoxes, this point retrogrades on the ecliptic. Consequently, it takes less time for the Moon to return to the equinox than it does to the same point between the fixed stars. Thus the tropical month is slightly shorter than the sidereal month.

Anomalistic month

This is the average time it takes for the Moon to pass from one perigee (the point in the lunar orbit where it is closest to Earth) to the next. An anomalistic month has an average duration of 27.55455 days.

Draconitic Month

The Moon's orbit is on a plane inclined to the ecliptic plane: it is tilted about five degrees. The line of intersection of this plane with the ecliptic defines two points on the celestial sphere: the ascending nodes and the descending node. These nodes are not fixed but rotate retrograde and give a full turn in approximately 18.6 years. The time it takes for the Moon to return to the same node is again shorter than a sidereal month (since the nodes meet it): this is called the draconitic month, and it lasts on average of approximately 27 1/5 days. It is important to predict eclipses: these take place when the Sun, Earth and Moon are in a line. Now (as seen from Earth) the Sun follows the ecliptic, while the Moon follows its own orbit, which is inclined. The three bodies are only in a line when the Moon is close to the elliptical, that is, when it is close to one of the nodes. The term draconitic refers to the mythological dragon that lives in the nodes and regularly eats the Sun or Moon during the eclipse.

The lengths of the astronomical months

The average duration of the different lunar months is not constant. Thus, next to the list, its secular linear variation is given.

Calendar month

Usually used in legal terminology, a calendar month is the time from the first day of a month to the last, inclusive. Therefore it can have a value of 28, 29, 30 or 31 days depending on the month in which the term is used.

Equivalences of other time units in months

• 30 days is a month
• One year is 12 months
• 5 years (light) is 60 months
• 10 years (decade) is 120 months
• 50 years is 600 months
• A century is 1200 months
• A millennium is 12 000 months